Structural evolution from preorganized mononuclear triazamacrocyclic metalloligands to polynuclear metallocages and heterometallic 2D layers: modular architectures, assembly tracking and magnetic properties

Abstract

The reaction of a macrocyclic ligand 1,4,7-triazacyclononane-1,4,7-tripropionic acid (tacntpH₃) with Ni(II)/Co(II) sources in the presence or absence of lanthanide cations yielded a series of doubly mononuclear ionic complexes (H₃O⁺)[Ln^III(H₂O)₈][Ni^II(tacntp)]₄ (1-NiLn, Ln = La, Ce, Yb) and a mononuclear Co(III) complex [Co^III(tacntp)]·4H₂O (2-Co) incorporating transition metal centers enveloped by both an azamacrocycle ring and pendant carboxylate groups. Either of the two preorganized mononuclear species [M^II/III(tacntp)]^−/0 was taken as a tripodal 3d metalloligand for the further assembly of modular architectures. Two pentanuclear metallocage-[Ln(NO₃)₆]³⁻ complexes [Ni^II₅(tacntp)₂(H₂O)₁₂][Ln^III(NO₃)₆]Cl·2H₂O (3-NiLn, Ln = La, Ce), one nonanuclear metallocage [Ni^II₉(tacntp)₄(H₂O)₁₈](ClO₄)₆·10H₂O (4-Ni), and two types of 2D layered heterometallic 3d–4f coordination networks ((H₃O⁺)[Ni^II₂Yb^III(tacntp)₂](ClO₄)₂·3H₂O (5-NiYb) and [Co^III₂Tb^III(tacntp)₂(H₂O)₃](ClO₄)·Cl₂·5H₂O (6-CoLn, Ln = La, Eu, Tb, Dy)) differing largely in their weaving architecture were controllably prepared via the precise regulation of multiple reaction parameters. Furthermore, the time-dependent evolution of metalloligand-derived species was tracked via electrospray ionization-mass spectrometry, to elucidate the modular assembly pathway for the nonanuclear Ni(II) metallocage 4-Ni. Analysis of the structural details and the assembly procedures of the modular architectures revealed that the pH, metalloligand size, and the type of metal bound to the metalloligand considerably influence the structural evolution from mononuclear metalloligands to cage-like and polymeric coordination architectures. Magnetic property studies disclosed that pendant syn–anti carboxylate groups favor a weak ferromagnetic exchange between the adjacent Ni(II) ions within the metallocages of 3-NiLa and 4-Ni but facilitate an antiferromagnetic coupling between the alternating Ni(II) and Yb(III) centers in 5-NiYb.